1. Field of the Invention
The present invention relates to a frequency modulation system which assigns different frequencies f.sub.0 and f.sub.1 to logical values 0 and 1 and generates signals having those two frequencies in a switching manner in accordance with the logical value of an input.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a system for frequency modulation according to the prior art. An oscillator 101 generating an output F.sub.0 of a frequency f.sub.0 corresponding to the logical value 0 has its output terminal connected to an input terminal 104 of a switching device 103, whereas an oscillator 102 generating an output F.sub.1 of a frequency f.sub.1 corresponding to the logical value 1 has its output terminal connected to an input terminal 105 of the switching device 103. Input data Din to the modulation system is applied directly to a switching input terminal 106 of the switching device 103. Output data D.sub.out from an output terminal 107 of the switching device 103 is applied to a filter 108 limiting its frequency band, the output of which becomes a modulated output M.sub.out.
The output signal F.sub.0 from the oscillator 101 and the output signal F.sub.1 from the oscillator 102 are not synchronized with each other. Because the input data D.sub.in to the modulation system is applied to the switching input terminal 106 of the switching device 103, the output of the switching device 103 is switched between signal F.sub.1 and signal F.sub.0 independently of the respective phases of signals F.sub.0 and F.sub.1 in accordance with changes in the input signal D.sub.in to the modulation system.
FIG. 2 illustrates the waveforms at different parts of the modulation system shown in FIG. 1. In FIG. 2, reference numerals 201 and 202 respectively show output signals F.sub.0 and F.sub.1 of the oscillators 101 and 102, numeral 203 the input signal D.sub.in, numeral 204 the output signal D.sub.out from the switching device 103, and numeral 205 the output signal M.sub.out from the filter 108. The signal waveforms of the different parts when an input data D.sub.in having the waveform illustrated by 203 in FIG. 2 is applied to the switching input terminal 106 of the switching device 103 are illustrated by 201, 202, 204, and 205. The input signal D.sub.in 203 changes from logical value 0 to logical value 1 at a time T.sub.1. The output signal from the switching device 103 is switched by that input data D.sub.in. More specifically, before time T.sub.1 signal F.sub.0, the waveform of which is illustrated by 201, is generated as the output signal 107 from the switching device 103, and after time T.sub.1 signal F.sub.1, the waveform of which is illustrated by 202, is generated. In other words, since the signals F.sub.0 and F.sub.1 and the input data D.sub.in to the input terminal 106 of the switching device 103 are not synchronized with each other, the output signal D.sub.out from the output terminal 107 of the switching device 103 has a waveform whose phase changes abruptly at time T.sub.1, as shown by 204. The modulated signal M.sub.out, which is obtained by filtering the output signal 204 through the filter 108, has a waveform whose amplitude and phase are very distorted in the vicinity of time T.sub.1, as shown by 205 in FIG. 2. Because of the incompleteness of this modulation system, the time during which there is an indefinite output during the interchange between the logical values 1 and 0 is extended in the prior art within a predetermined band width at the demodulator, so that the bit rate obtainable is lower than the theoretical value determined by the band width. In other words, in order to obtain a predetermined bit rate and bit error rate, it is necessary in the prior art to enlarge the band width of the transmission line sufficiently more than the theoretical value.